Crosstalk Reduction Strategies on UCPW Transmission Lines Supported by Real Application Data: Experimental Analysis of Point Capacity

Abstract

Crosstalk between transmission lines, primarily caused by capacitive coupling, is a major challenge in high-frequency electronic systems, leading to signal integrity degradation. This study investigates the effectiveness of capacitors placed between ground planes in ungrounded coplanar waveguide (UCPW) transmission lines fabricated on FR4 circuit boards. A vector network analyzer (VNA) was used to measure near-end crosstalk (S₃₁) reduction, with improvements of up to −40 dB observed. Experiments were conducted on transmission lines of 100 mm and 200 mm lengths, demonstrating the impact of capacitor placement on mitigating interference. The results indicate that this method provides a scalable and practical approach to improving signal integrity in compact, high-density electronic designs. These findings contribute to a deeper understanding of crosstalk mitigation strategies, offering valuable insights for applications in high-speed communication and RF circuit design. This work systematically analyzes the role of capacitor placement in reducing crosstalk, addressing a critical gap in the literature and paving the way for future advancements in transmission line optimization.

1. Introduction

The performance of modern electronic communication systems is critically influenced by signal transmission quality, bandwidth efficiency, and design optimization [1,2,3]. Ensuring accurate transmission and protecting high-frequency signals are essential for maintaining signal integrity and overall system stability. However, electromagnetic interactions between transmission lines can compromise these objectives, with crosstalk emerging as a significant challenge [4]. Crosstalk, caused by unintended electromagnetic coupling, is particularly problematic in high-frequency transmissions, where it degrades signal integrity and diminishes the stability of transmission lines [5,6,7,8].

Crosstalk effects are especially pronounced in sensitive electronic systems, such as high-frequency microprocessors and communication devices. In such systems, signal distortions often lead to data errors and operational instability, resulting in performance loss or malfunctions observable by users. Furthermore, these distortions exacerbate electromagnetic compatibility (EMC) challenges, complicating the seamless operation of interconnected systems [9,10,11].

As electronic circuits evolve toward increased miniaturization, the demand for compact designs grows alongside performance expectations [12,13,14]. UCPW transmission lines have emerged as an efficient solution for high-frequency applications. Their low-loss characteristics and simplified design eliminate the need for a ground plane beneath the signal line, making them cost-effective and easier to manufacture compared to traditional coplanar waveguide (CPW) structures [15,16,17]. UCPW configurations allow parallel transmission lines to be positioned on the same plane in close proximity, making them ideal for RF and microwave applications. Moreover, UCPW’s compact design and low radiation losses enhance its utility in high-frequency circuits, antennas, and communication systems [18].

However, traditional methods for reducing crosstalk, such as increasing the spacing between transmission lines or using guard traces, face significant challenges in high-density PCB designs due to space and cost constraints. While CPW lines are extensively studied, the impact of integrating capacitors between ground planes in UCPW configurations remains underexplored. Addressing this gap is crucial for advancing high-frequency communication systems.

The integration of passive components, particularly capacitors, between reference planes has been extensively studied to improve transmission line performance. Capacitors create low-impedance return current paths, which enhance signal integrity and suppress electromagnetic interference [19,20,21]. Decoupling capacitors are particularly effective in reducing noise and stabilizing the performance of high-frequency transmission lines [22,23,24,25].

This study investigates whether the inclusion of capacitors between ground planes in a UCPW-configured two-layer circuit board with 50 Ω impedance can effectively reduce crosstalk and preserve signal integrity in high-frequency applications. By systematically analyzing the effects of capacitors on near-end crosstalk (S₃₁) in UCPW transmission lines, this study provides a novel perspective on mitigating interference and enhancing performance. Building on these insights, the proposed method not only achieves significant improvements in signal integrity but also offers a practical and scalable solution for high-density electronic systems, effectively bridging a critical gap in the existing research. This correlation highlights the method’s ability to improve signal integrity, ensure practical scalability, and address a critical research gap.

2. Theoretical Infrastructure

The theoretical considerations in this section provide a crucial foundation for ensuring impedance matching in the experimental setup, which is essential for accurately evaluating crosstalk effects. Maintaining consistent impedance along the transmission lines minimizes unintended signal reflections, thereby improving the reliability of the measurements. In this context, the impedance calculations follow the well-established methodologies outlined by Wadell [16], which are widely used in transmission line design and grounded in fundamental electromagnetic theory. These methodologies are consistent with commonly accepted industry practices and have been extensively validated in the literature for coplanar waveguide structures. While they may not be explicitly codified in formal standards such as IEEE or IPC, they are derived from rigorous theoretical principles and experimental validations, making them a reliable basis for this study.

2.1. UCPW Transmission Lines and Key Feature

Coplanar waveguide transmission lines typically consist of a signal line flanked by ground planes on the same layer, with some configurations including an additional ground plane beneath the signal line. Ungrounded coplanar waveguide transmission lines differ by omitting this lower ground plane, resulting in distinct electromagnetic characteristics. This simplified design is particularly advantageous for compact and cost-sensitive high-frequency circuits. Figure 1 illustrates the UCPW configuration, where key parameters include the line width (w), spacing between the signal line and ground planes (s), copper thickness (t), dielectric height (h), and dielectric constant (ε_r).

Maintaining consistent impedance along transmission lines is crucial for preserving signal integrity. Impedance mismatches can cause signal reflections, losses, and parasitic interactions that degrade overall performance. The reflection coefficient used to evaluate these mismatches is calculated by Equation (1) [15].

$$\mathsf{\rho }={\displaystyle \frac{{\mathrm{V}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}}}{{\mathrm{V}}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{t}}}}={\displaystyle \frac{{\mathrm{Z}}_{\mathrm{s}}-{\mathrm{Z}}_{\mathrm{i}\mathrm{n}}}{{\mathrm{Z}}_{\mathrm{s}}+{\mathrm{Z}}_{\mathrm{i}\mathrm{n}}}},$$

(1)

where V_reflected and V_incident are the reflected and incident voltages, Z_in is the instantaneous impedance at the signal entry point, and Z_s represents the initial impedance.

When stacking layers in a PCB design, precise control over the distance between layers is essential. Parameters such as the dielectric material type, its constant ε_r, and the spacing between the ground and signal layers all play critical roles in determining transmission line impedance. The stack design must consider material choices, layer thicknesses, and insulating properties, which collectively impact circuit performance and signal integrity [26,27,28].

In UCPW transmission lines, the impedance calculation follows a structured series of steps outlined by Wadell [16]. This process initiates with determining the transmission line width and the distance to the ground plane. Subsequently, Equations (2) through (10) are applied, accounting for the effects of copper thickness on the variables a and b. These parameters are defined as follows:

In UCPW transmission lines, the impedance calculation follows a structured series of steps outlined by Wadell [16].

a = w,

(2)

b = w + 2s,

(3)

$${\mathrm{a}}_{\mathrm{t}}=\mathrm{a}+{\displaystyle \frac{1.25\mathrm{t}}{\mathsf{\pi }}}\left(1+\mathrm{l}\mathrm{n}\left({\displaystyle \frac{4\mathsf{\pi }\mathrm{a}}{\mathrm{t}}}\right)\right),$$

(4)

$${\mathrm{b}}_{\mathrm{t}}=\mathrm{b}-{\displaystyle \frac{1.25\mathrm{t}}{\mathsf{\pi }}}\left(1+\mathrm{l}\mathrm{n}\left({\displaystyle \frac{4\mathsf{\pi }\mathrm{a}}{\mathrm{t}}}\right)\right),$$

(5)

The resulting parameter k is derived from a and b, and k_t is similarly computed using a_t and b_t:

$$\mathrm{k}={\displaystyle \frac{\mathrm{a}}{\mathrm{b}}},$$

(6)

$${\mathrm{k}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{a}}_{\mathrm{t}}}{{\mathrm{b}}_{\mathrm{t}}}},$$

(7)

$${\mathrm{k}}_{\mathrm{t}}^{\prime }=\sqrt{1-{\mathrm{k}}_{\mathrm{t}}^2},$$

(8)

An additional variable k₁ is calculated based on hyperbolic sine functions of at and b_t relative to dielectric height h:

$${\mathrm{k}}_1={\displaystyle \frac{\sin \mathrm{h}\left(\frac{\mathsf{\pi }{\mathrm{a}}_{\mathrm{t}}}{4\mathrm{h}}\right)}{\sin \mathrm{h}\left(\frac{\mathsf{\pi }{\mathrm{b}}_{\mathrm{t}}}{4\mathrm{h}}\right)}},$$

(9)

$${\mathrm{k}}_1^{\prime }=\sqrt{1-{\mathrm{k}}_1^2},$$

(10)

With the calculated k values, the elliptic integral K(k) is determined as follows:

$$K\left(\mathrm{k}\right)={\int }_0^{{\displaystyle \frac{\mathsf{\pi }}{2}}}{\displaystyle \frac{\mathrm{d}\mathsf{\Phi }}{\sqrt{1-{\mathrm{k}}^2{\sin }^2\mathsf{\Phi }}}},$$

(11)

Using K(k) and dielectric constant ε_r, the effective dielectric constant ε_eff is given by Equation (12).

$${\mathsf{\epsilon }}_{{\mathrm{e}}_{\mathrm{f}\mathrm{f}}}=1+{\displaystyle \frac{{\mathsf{\epsilon }}_{\mathrm{r}}-1}{2}}{\displaystyle \frac{\mathrm{K}\left({\mathrm{k}}^{\prime }\right)}{\mathrm{K}\left(\mathrm{k}\right)}}{\displaystyle \frac{\mathrm{K}\left({\mathrm{k}}_1\right)}{\mathrm{K}\left({\mathrm{k}}_1^{\prime }\right)}},$$

(12)

Finally, after accounting for copper thickness, the modified effective dielectric constant ε_eff,t and characteristic impedance Z₀ are calculated with Equations (13) and (14).

$${\mathsf{\epsilon }}_{{\mathrm{e}}_{\mathrm{f}\mathrm{f},\mathrm{t}}}={\mathsf{\epsilon }}_{\mathrm{e}\mathrm{f}\mathrm{f}}-{\displaystyle \frac{{\mathsf{\epsilon }}_{{\mathrm{e}}_{\mathrm{f}\mathrm{f}}}-1}{\frac{\left(\mathrm{b}-\mathrm{a}\right)∕2}{0.7\mathrm{t}}\frac{\mathrm{K}\left(\mathrm{k}\right)}{{\mathrm{K}}^{\prime }\left(\mathrm{k}\right)}+1}},$$

(13)

$${\mathrm{z}}_0={\displaystyle \frac{30\mathsf{\pi }}{\sqrt{{\mathsf{\epsilon }}_{\mathrm{e}\mathrm{f}\mathrm{f},\mathrm{t}}}}}{\displaystyle \frac{\mathrm{K}\left({\mathrm{k}}_{\mathrm{t}}^{\prime }\right)}{\mathrm{K}\left({\mathrm{k}}_{\mathrm{t}}\right)}},$$

(14)

2.2. An Overview of Crosstalk Reduction Method

Crosstalk, caused by undesired electromagnetic coupling between transmission lines, is a significant challenge in transmission systems. Effective reduction techniques often involve optimizing physical configurations and employing material solutions to minimize these interactions.

One of the most straightforward methods to reduce crosstalk is adjusting the physical layout of transmission lines. Increasing the distance between adjacent lines or cross-positioning parallel lines can help mitigate electromagnetic coupling [29,30]. Insulation materials with high dielectric properties placed between layers are also commonly used to suppress capacitive coupling, further reducing crosstalk [31].

In UCPW structures, where transmission lines are flanked by ground planes, crosstalk can be further mitigated by creating a localized return current path. Adding capacitors between these ground planes provides a low-impedance path for the return current, reducing the interaction between signal lines. While capacitors primarily affect the return current flow, their presence can influence the overall coupling by stabilizing the electromagnetic environment between the transmission lines [32,33,34].

The crosstalk effects are typically evaluated using two key parameters: NEXT (near-end crosstalk, S₃₁) and FEXT (far-end crosstalk, S₄₁). Among the commonly used formulations for these parameters, Equations (15) and (16) are particularly relevant [35].

$${\mathrm{N}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\displaystyle \frac{1}{4}}\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{m}}}{\mathrm{C}}}+{\displaystyle \frac{{\mathrm{L}}_{\mathrm{m}}}{\mathrm{L}}}\right),$$

(15)

$${\mathrm{F}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\displaystyle \frac{1}{2\mathrm{v}}}\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{m}}}{\mathrm{C}}}-{\displaystyle \frac{{\mathrm{L}}_{\mathrm{m}}}{\mathrm{L}}}\right)$$

(16)

In these equations, C_m and L_m represent the mutual capacitance and inductance between transmission lines, while C and L denote the per-unit-length capacitance and inductance of a single line. The signal propagation speed is represented by v. Both equations highlight the crucial role of mutual coupling parameters, where increased mutual capacitance or decreased mutual inductance intensifies crosstalk effects. To mitigate this, capacitors placed between ground planes establish a controlled return current path, effectively confining the current within a localized loop and minimizing interference with the signal lines. This controlled return path reduces overall mutual coupling and stabilizes both NEXT (S₃₁) and FEXT (S₄₁) parameters, as also modeled in prior studies on coupled RLC interconnects [35]. A comparable analysis of mutual coupling based on electromagnetic field distribution is also provided by Xu [36], who investigates crosstalk suppression in photonic and microwave integrated circuits using field-theoretical models.

Although capacitors do not directly reduce mutual capacitance (C_m), they play a fundamental role in modifying the electromagnetic field distribution between transmission lines. By directing the return current through a well-defined path, capacitors minimize unwanted coupling, thereby improving signal integrity in UCPW transmission lines. The experimental observations in this study align with theoretical formulations of mutual coupling behavior, demonstrating that strategic capacitor placement significantly improves crosstalk suppression. This approach not only mitigates coupling effects but also preserves impedance matching, making it a viable solution for high-speed and high-density electronic applications.

While capacitors may not directly lower the value of mutual capacitance (C_m), they significantly influence the electromagnetic field distribution between transmission lines. By guiding the return current through a well-defined path, capacitors reduce the potential for interference and enhance the crosstalk performance of UCPW structures. This approach not only mitigates coupling but also maintains impedance matching, making it particularly effective for high-speed transmission systems.

The significance of this method lies in its ability to enhance signal integrity in high-frequency transmission systems while maintaining structural simplicity. Unlike conventional shielding techniques, which often introduce additional complexity and material constraints, the capacitor-based approach provides a more efficient means of crosstalk mitigation by establishing a controlled return current path. This not only reduces electromagnetic interference but also stabilizes signal propagation without necessitating major modifications to the transmission line structure. Additionally, its seamless integration into existing UCPW configurations enhances its practical applicability, making it a scalable and cost-effective solution for improving signal integrity in high-density electronic systems.

2.3. Potential Effect of Capacitors on Crosstalk

Capacitors significantly influence crosstalk between transmission lines, especially when placed between reference planes. By creating alternative paths for return currents, capacitors help reduce mutual coupling and electromagnetic interactions during signal transmission [37]. This design strategy is particularly effective in maintaining signal integrity in high-speed systems.

However, the impact of capacitors is not exclusively beneficial. Improperly placed or sized capacitors can lead to unwanted resonances or frequency-dependent anomalies, potentially amplifying crosstalk rather than mitigating it. This phenomenon occurs when the capacitors create unintended resonant circuits, disrupting the desired signal flow and increasing electromagnetic coupling.

To ensure capacitors have a positive effect, they must be carefully selected and configured. The capacitance value, placement, and spacing relative to the signal frequency and transmission line geometry are critical parameters. When properly optimized, capacitors reduce coupling effects, preserve impedance matching, and enhance the overall electromagnetic performance of the transmission system. On the contrary, inadequate design or improper placement can compromise the system’s performance by introducing additional interference.

2.4. Recommended Transmission Line Structure

As shown in Figure 2, two ungrounded coplanar transmission lines are routed side by side. Unlike traditional methods that place capacitors directly between signal lines or between signal lines and ground planes, capacitors, designated as C_first, C_middle, and C_end, are positioned exclusively between the ground planes at specified locations. Each capacitor is soldered onto pre-designed pads placed along the ground lines. This configuration ensures that the capacitors influence the return current path rather than directly interacting with the signal traces, which helps maintain signal integrity without introducing unwanted resonance effects. These pads are strategically distributed to enable flexible positioning of capacitors, allowing for the evaluation of various capacitance values and configurations to achieve optimal crosstalk reduction in unique designs.

The capacitors were soldered manually onto these pads using standard high-frequency PCB assembly techniques. This approach ensures precision and repeatability in capacitor placement. Additionally, the inclusion of these pads during the PCB design phase provides the flexibility to adjust the placement or replace capacitors without requiring a redesign or re-fabrication of the PCB. This post-production adjustability represents a significant advantage over other methods, particularly in iterative design environments or for prototyping purposes.

In this setup, Transmission Line 1 is defined as the victim line, and Transmission Line 2 is designated as the aggressor. Termination resistors with values of 50 Ω are used for impedance matching at high frequencies, ensuring accurate S-parameter measurements. The transmission line dimensions used in this study are as follows: w = 0.14 mm, s = 0.1 mm, t = 0.035 mm, h = 1.5 mm, and ɛ_r = 4.5 F/m. With these parameters, the designed transmission line achieves an approximate impedance of 50 Ω, ensuring compatibility with high-frequency applications. The dielectric material for the circuit board is selected as FR4 due to its cost-effectiveness and widespread usage in high-frequency PCB designs.

By enabling precise placement and post-production adjustments of capacitors, this approach offers an effective, scalable, and practical method for reducing crosstalk in high-frequency transmission lines. Unlike conventional approaches where capacitors can create frequency-dependent resonance effects, the placement of capacitors strictly between the ground planes prevents unwanted filtering artifacts. The flexibility of this design ensures its adaptability to a wide range of applications, addressing both experimental and production-level requirements.

As depicted in Figure 2, the capacitors are specifically placed between the ground planes rather than between signal traces. This key distinction ensures that the capacitors primarily act on the return current flow, rather than introducing direct reactive effects into the signal path. By carefully optimizing their placement, the capacitors enhance electromagnetic shielding while preserving impedance consistency across the transmission line.

3. Materials and Methods

In this study, the crosstalk levels at various frequency ranges were measured to evaluate the mutual interactions between signals. The conducted measurements aimed to quantify the extent of contamination caused by electromagnetic coupling. Tests were performed using a two-layer circuit board with two different transmission line lengths. The choice of varying line lengths was deliberate, enabling the observation of how line length influences the effectiveness of capacitors placed between ground planes.

The methodology of this study integrates multiple techniques, including the arrangement of signal paths, strategic component placement, shielding, and electromagnetic interference-based crosstalk measurement. These techniques specifically target the reduction of near-end crosstalk (S₃₁).

The circuit board designs were created using Altium Designer 21.5.1 software. The scenario of routing UCPW lines side by side was tested to simulate realistic high-frequency transmission environments. High-quality components were selected for assembly to ensure measurement accuracy. For crosstalk testing, a Siglent SSA3032X Plus model VNA (SIGLENT Technologies, Shenzhen, China) was employed. The VNA was calibrated using F503ME series calibration kits prior to testing, ensuring measurement precision.

Capacitors from Johanson Technology’s designer kits (Johanson Technology, Camarillo, CA, USA) were utilized during the tests, specifically the S603DS series, which features a self-resonant frequency (SRF) of approximately 20 GHz. The SRF of these capacitors is critical, as it determines their effectiveness in mitigating high-frequency interference. Termination resistors compatible with the operational frequency range were used on the signal-applied lines to maintain impedance matching and minimize reflections. For this study, Eastsheep 2W-SMA-JR-12G resistors (Eastsheep, Beijing, China), compatible up to 12 GHz, were chosen.

To eliminate environmental factors that could affect test accuracy, all measurements were conducted under controlled conditions. Both circuit boards were housed inside grounded metal enclosures to shield them from external electromagnetic interference, ensuring consistent and reproducible test results. Prior to each measurement session, the VNA was calibrated using the F503ME series calibration kit with Short, Open, Load, and Through (SOLT) standards. This calibration method corrects systematic errors, ensuring accurate impedance matching and reliable S-parameter measurements across the tested frequency range. The procedure followed the manufacturer’s recommended guidelines to minimize measurement uncertainties. The test environment is depicted in Figure 3.

A capacitor is placed between the ground planes flanking the transmission line. The positions of these capacitors are strategically selected as the starting point, midpoint, and endpoint of the segment where CPW traces run in parallel. These positions were chosen to evaluate the effect of capacitors on electromagnetic coupling at different points along the transmission line. Additionally, the lengths of the transmission lines are set to progress together at 100 mm and 200 mm, allowing for a comparison of crosstalk behavior across different line lengths. The physical structure of the test card and the placement of passive components are shown in Figure 4.

To systematically evaluate the influence of capacitors, repeated tests were conducted by fixing the values of the capacitors added to the UCPW traces on the circuit board. The selection of capacitor values (1 pF, 15 pF, and 82 pF) was determined based on practical applicability in high-frequency PCB designs and compatibility with industry-standard component kits. While numerous capacitance values exist, high-frequency performance is highly dependent on the SRF rather than the absolute capacitance alone. The selected values represent low, medium, and high capacitance ranges commonly used in RF circuit design and allow for a comparative evaluation of their impact on crosstalk reduction. Additionally, these values were chosen to align with widely available RF capacitor kits, ensuring reproducibility and practicality in real-world implementations. This selection method enables a structured experimental approach rather than arbitrary testing, ensuring that the findings align with realistic engineering constraints.

4. Experimental Result

The results obtained in this study evaluate the effectiveness of various methods for mitigating crosstalk effects in CPW lines. The measurements clearly demonstrate the presence of crosstalk between CPW lines, particularly at high frequencies. The addition of capacitors to the ground (GND) planes was found to significantly reduce crosstalk at specific frequencies. These capacitors provide an alternative path for return currents, thereby stabilizing the electromagnetic environment and reducing mutual coupling between transmission lines.

As mentioned in the Section 3, the capacitors used in this study belong to the S603DS series and have an SRF of approximately 20 GHz. This frequency range ensures their effectiveness in mitigating crosstalk in high-speed applications. By providing effective performance up to 20 GHz, these capacitors are well suited for practical high-frequency designs, as verified in this study. The selected capacitor values of 1 pF, 15 pF, and 82 pF were chosen for their availability and practicality in real-world applications.

Additionally, the spacing between transmission lines was shown to have a substantial impact on crosstalk levels. The measurements indicate that crosstalk increases at shorter distances due to stronger electromagnetic coupling, whereas greater spacing reduces this effect. These findings underscore the importance of optimizing line spacing during the design phase to control crosstalk in electronic circuits.

Table 1. Optimum near-end crosstalk improvements with capacitors added between 3W wide ground planes.

Length (mm)	Value (pF)	Location	Frequency (GHz)	Improvement (−dB)
100	82	First	2.7	25
200	15	First	2.9	35

and

Table 2. Optimum near-end crosstalk improvements with capacitors added between 5W wide ground planes.

Length (mm)	Value (pF)	Location	Frequency (GHz)	Improvement (−dB)
100	15	End	2.1	25
200	82	End	1.5	40

summarize the improvements in S₃₁ achieved by adding capacitors to the GND planes for two different UCPW line designs fabricated using FR4 dielectric material. The selection of 3W and 5W as ground plane widths is based on design guidelines provided by semiconductor manufacturers and industry standards for high-speed PCB design. ‘W’ represents the transmission line width, meaning that 3W and 5W correspond to ground planes that are three and five times the width of the signal line, respectively.

While 1W spacing was theoretically an option, it was excluded due to its limited effectiveness in practical applications. Manufacturer guidelines and industry best practices recommend 3W and 5W configurations, as they provide an optimal balance between manufacturability, crosstalk reduction, and signal integrity. Furthermore, validation tests and performance benchmarks are typically conducted based on these ratios, reinforcing their relevance in real-world designs.

Analysis of the data in Table 1 and Table 2 reveals that capacitors added to the reference planes between the transmission lines significantly reduce crosstalk as the length of the transmission lines increases. The observed improvements also depend on the width of the ground plane, with reductions noted for both 3W and 5W configurations. These findings confirm the critical role of capacitor placement and reference plane dimensions in control-ling crosstalk. The frequency values presented in the tables correspond to the points where the highest crosstalk reduction was observed for each configuration. Rather than evaluating arbitrary frequency points, the selection was based on the frequency ranges where capacitive coupling effects provided the most effective mitigation. This approach ensures that the results highlight the practical effectiveness of capacitor placement within relevant high-frequency operating conditions.

This study also highlights the practical advantages of this approach in real-world applications. By enabling post-production integration of capacitors, this method allows for system optimization without the need for redesigning or re-fabricating the circuit board. Such flexibility is particularly beneficial in iterative design environments and high-frequency communication systems, where reducing crosstalk is critical for maintaining signal integrity.

This research presents test results for cases where the ground plane between transmission lines is three times the width of the transmission line (3W) and five times the width (5W). In the experiments, capacitors located at the starting, middle, and ending points of the sections where the transmission lines progress together were evaluated. These positions are indicated by the codes F (First), M (Middle), and E (End).

Capacitor values of 1 pF, 15 pF, and 82 pF were tested separately for each position and compared with the baseline condition (no capacitors). The test results are presented in graphical format, with the baseline condition shown in black for clarity. To facilitate interpretation, color coding was applied to the graphs, indicating the specific capacitor values and positions. The color codes and corresponding scenarios are detailed in Figure 5.

This study provides valuable insights into the effects of various ground plane widths and capacitor placements on the capacitive properties of transmission lines in circuit boards fabricated with FR4 dielectric material. Crosstalk measurements obtained using a VNA are presented graphically to illustrate the impact of capacitors on crosstalk reduction under different configurations. The test results include scenarios where capacitors are located at the beginning, midpoint, and endpoint of the transmission line segments. Additionally, the influence of line length was systematically evaluated by comparing results for transmission line lengths of 100 mm and 200 mm, where the lines run parallel. These findings represent a significant step towards optimizing high-frequency transmission line designs by effectively reducing crosstalk and enhancing signal integrity.

4.1. The VNA Graphics of Tests Performed on 3W Ground Plane Transmission Line

This section presents the measurement results obtained from a transmission line model with a ground plane width three times the width of the signal line (3W). The experiments primarily focused on analyzing near-end crosstalk (S₃₁). The effects of capacitors placed at different positions along the 100 mm transmission line are depicted in Figure 6. Specifically, Figure 6a illustrates the results when the capacitor is located at the starting point, Figure 6b corresponds to the midpoint placement, and Figure 6c represents the endpoint configuration.

As illustrated in Figure 2, the placement of capacitors at different positions along the transmission line directly influences the return current path, thereby affecting crosstalk mitigation. When the capacitor is placed at the beginning of the transmission segment in Figure 6a, it primarily influences the initial propagation of the return current and modifies the early-stage coupling behavior. The middle placement Figure 6b distributes the capacitive effect along the transmission length, promoting a more uniform reduction in crosstalk. In contrast, placing the capacitor at the end of Figure 6c mainly affects the termination region, where secondary coupling effects and reflections can be more significant. These placements allow for a comparative evaluation of how capacitor positioning influences crosstalk suppression along different sections of the transmission line.

In Figure 6, the effects of capacitors placed at different positions along the 100 mm transmission line are illustrated. The observed impact varies based on the frequency and capacitance values. For instance, when an 82 pF capacitor was placed at the beginning of the transmission line, a crosstalk reduction of approximately 20–25 dB was achieved within the 1.6–2.7 GHz frequency range. When capacitors were added at the midpoint of the transmission line, all tested values showed improvements starting from 1.9 GHz. Notably, when an 82 pF capacitor was placed at the endpoint of the transmission line, an improvement of approximately 22 dB was observed. Furthermore, in the frequency range of 2.5–2.85 GHz, the 1 pF capacitor exhibited superior performance compared to the other tested capacitance values.

The effects of capacitors on the 200 mm transmission line are shown in Figure 7, with Figure 7a representing the beginning, Figure 7b the midpoint, and Figure 7c the endpoint placements.

As shown in Figure 7, the placement of capacitors on the 200 mm transmission line significantly impacts crosstalk reduction, depending on the frequency and capacitance values. When a 1 pF capacitor is placed at the beginning of the line, an improvement of 25 dB is observed around 1.1 GHz, while an 82 pF capacitor achieves the same level of improvement near 3 GHz. For capacitors placed at the midpoint of the line, all values exhibit positive effects between 2.9 GHz and 3.1 GHz, with 1 pF and 15 pF capacitors showing particularly strong performance. At the endpoint, adding a 15 pF capacitor results in a 30 dB improvement around 3 GHz.

4.2. The VNA Graphics of Tests Performed on 5W Ground Plane Transmission Line

In transmission line analysis, identifying aggressor and victim lines is essential for understanding crosstalk behavior. The aggressor line is the one to which the signal is applied, emitting noise and interference to surrounding lines, while the victim line is affected by this interference and maintains its current state.

This section presents measurement findings for a transmission line with a ground plane width five times that of the signal line (5W). To minimize the effects of production errors and variations in electronic circuit boards, experiments were conducted in different regions on the same panel. Similar to the previous section, measurements were performed for transmission line lengths of 100 mm and 200 mm. These experiments specifically focused on S₃₁, which plays a critical role in the performance and stability of the transmission line.

The capacitors were placed at three positions along the 100 mm transmission line to evaluate the effects of capacitive interactions on signal integrity. The results are presented in Figure 8, with Figure 8a showing the beginning placement, Figure 8b the midpoint placement, and Figure 8c the endpoint placement.

According to the graphs in Figure 8, the placement of capacitors significantly impacts crosstalk reduction on the 100 mm transmission line. At the beginning of the line, 15 pF and 82 pF capacitors demonstrate improvements between 1.3 GHz and 1.6 GHz. For capacitors placed at the midpoint, all values show positive effects around 1.6 GHz, with 15 pF and 82 pF providing improvements of up to 20 dB. At the endpoint, 15 pF and 82 pF capacitors similarly achieve up to 20 dB improvements in the 1.3–1.6 GHz range. Notably, the 15 pF capacitor achieves a peak improvement of 25 dB around 2.1 GHz.

For the 200 mm transmission line model, the capacitor placements are presented sequentially in Figure 9, with Figure 9a showing results for the beginning placement, Figure 9b for the midpoint, and Figure 9c for the endpoint.

According to the graphs in Figure 9, capacitors placed at different positions on the 200 mm transmission line exhibit significant crosstalk reduction effects across various frequency ranges. At the beginning of the line, 15 pF and 82 pF capacitors provide an improvement of approximately 25 dB around 1.9 GHz, while a 1 pF capacitor achieves an improvement of 30 dB around 2.4 GHz. When capacitors are placed at the midpoint of the line, 15 pF and 82 pF capacitors deliver improvements of up to 30 dB across nearly the entire range of 1.35 GHz to 2.5 GHz.

At the endpoint, 15 pF and 82 pF capacitors provide improvements over a wide range from 650 MHz to 3.1 GHz, with enhancements reaching up to 40 dB. Additionally, the 1 pF capacitor demonstrates improvements of up to 35 dB in the range of 2.9 GHz to 3.1 GHz. These results highlight the importance of both capacitor selection and placement in mitigating crosstalk and maintaining signal integrity across diverse frequency bands.

5. Discussion and Conclusions

This study demonstrates that adding capacitors within the ground planes is an effective method for reducing crosstalk between UCPW lines. The proposed method of placing capacitors between ground planes rather than between signal lines offers a viable and efficient alternative for engineers, particularly in high-density PCB designs where space constraints are critical.

The experimental results indicate S₃₁ improvements of up to 40 dB, as shown in Figure 7 and Figure 9. Additionally, improvements of 20–25 dB were observed in Figure 6 and Figure 8. These findings highlight the significant impact of capacitor placement and value on crosstalk mitigation. While the effect varies with frequency and positioning, the capacitors demonstrate their potential to effectively reduce interference within specific frequency bands. It should be noted that the measurements were conducted using the Siglent SSA3032X Plus VNA, which has an amplitude accuracy of ±0.7 dB, as specified in its datasheet. To enhance measurement stability and mitigate the effects of random noise and transient variations, the VNA was configured to perform an average of 100 sweeps per measurement. This averaging process reduces short-term fluctuations and improves result consistency; however, the fundamental measurement uncertainty remains governed by the inherent amplitude accuracy of ±0.7 dB. Therefore, the reported S₃₁ improvements should be interpreted within this measurement uncertainty range. This tailored performance makes the proposed method a practical solution for diverse high-frequency applications.

The findings of this study underscore the practical and technical advantages of the proposed method. Compared to the literature shown in

Table 3. Optimum near-end crosstalk improvements obtained in literature and this study.

Study	Method	Optimum S31 Improvement (−dB)
Huang et al. [38]	Vias of serpentine guard trace	30
Suntives et al. [39]	Updating the number of vias to 25	15
Almalkawi et al. [40]	Fourier-based non-uniform victim line	25
Shim [41]	Dogbone via structure	15
Lee [42]	Serpentine split structure of ground planes	30
Wang et al. [30]	Making the via angle 45 degrees	6
This study	Adding capacity between ground planes	40

, methods such as those involving guard traces or perforated vias, the addition of capacitors exclusively within the ground plane structure provides significant improvements while avoiding increased circuit size, production complexity, and additional costs. Furthermore, the ability to integrate capacitors post-production offers flexibility in iterative design processes and high-frequency applications, where maintaining signal integrity is crucial.

While the proposed method demonstrates strong performance, further research could expand its applicability. Future studies could investigate its performance across different dielectric materials, explore its effects in multi-layered or more complex PCB designs, and assess its scalability for larger systems. These directions would help establish a broader foundation for implementing this method in diverse electronic applications.

Since this study was conducted using FR4 as the PCB substrate, its applicability to other materials with different dielectric constants and loss characteristics remains an open question. The effectiveness of capacitors in reducing crosstalk may vary depending on the electrical properties of alternative substrates such as Rogers, Teflon-based laminates, or other high-frequency PCB materials. If different dielectric materials are used, impedance calculations must be re-evaluated following the methodologies outlined in this study to ensure optimal crosstalk reduction and signal integrity. Future research should examine these variations to further validate the generalizability of the proposed approach.

In conclusion, the method presented in this study provides a robust and innovative solution for near-end crosstalk reduction. By achieving improvements of up to 40 dB, it outperforms many existing methods in terms of both effectiveness and practicality.